Docs site: brand.yml styling for the Quarto / pkgdown / TypeDoc pages

[schloerke]

Follow-up to #86. The docs site (Quarto landing + quartodoc at /py/, pkgdown at /r/, TypeDoc at /js/) ships with stock themes: Quarto cosmo, pkgdown Bootstrap 5 default, TypeDoc default.

Give it one look across all three:

• Add _brand.yml (colors, fonts, logo/shiny-react.svg) and switch pkg-py/docs/_quarto.yml to theme: [brand] (shinychat does this)
• Point pkg-r/_pkgdown.yml at the same brand file (template: brand: is supported by pkgdown ≥ 2.1)
• TypeDoc: customCss matching the brand palette, or accept the default
• Logo in the Quarto navbar / pkgdown home
• Set the repo homepageUrl to https://posit-dev.github.io/shinyreact/ once the first deploy lands